Another undocumented feature of the HRC has been uncovered recently.
First, some background information. In addition to the primary science
data for individual events, the rate of microchannel plate triggers
(total rate) and triggers that pass on-board validity
tests (valid rate)
are telemetered to the ground. The valid rate is used to
correct the primary rate for deadtime and telemetry saturation
effects. As long as the primary rate is below saturation, the
primary rate itself can be used to make the small
(1%) correction,
since the event processing dead-time is known. However, when the event
rate exceeds saturation (a not uncommon occurrence because of the flaring
background from low energy protons) the valid rate is necessary to correct
the event rate. Unfortunately, the total and valid event rates are
overestimated by about 15% for normal operation of the HRC-S . In the
HRC-S ``imaging" (aka ``timing") mode these rates are overestimated by a
few percent, at most. The problem is caused by an overshoot
in occasional large trigger pulses.
This results in double counting in the total and valid event
on-board scalers. The primary science event is not affected,
since once event processing starts with the initial trigger
pulse, a gate rejects further pulses until processing is complete.
The HRC-I does not have the overshoot problem. We suspect that rise-time
differences in the trigger pulses in I and S are the reason for the
difference in the responses. We are studying ways of mitigating the
problem. Raising the trigger threshold level reduces the double
counting but we need to evaluate the consequence on detector QE
(quantum efficiency)
and uniformity. The simplest and least ``invasive" solution would
be to apply a correction factor based on the fraction of large
pulses determined from the pulse height distribution obtained
during an observation.
We are studying this solution.

The HRC remains robust and healthy. Detector temperatures and
voltages remain nominal. The HRC-I temporal stability has been
examined and a longer baseline will be required to perceive
any temporal drift in the QE.
There is, however, a clear,
slight downward drift in the HRC-I gain of about 2% per year.
In the future, if this trend continues and affects performance,
the gain can be raised by raising the detector voltage.

Figure 5:
An HRC image of the central region of Cas A. The central point source is in the center of the image. The double arrow is 20 arcsec wide.

X-ray observations of the supernova remnant Cas A using the
Einstein, ROSAT and ASCA observatories failed to detect a central point
source. However, with the 6000 second ``first light" observation of
Cas A by ACIS, a central point source ``stuck out like a sore thumb".
The object is likely to be a neutron star or black hole that remained
after the explosion of the progenitor of the supernova; efforts to
find a radio or optical counterpart have failed so far. Surmising
that a rotating neutron star would produce periodic pulses, a team
led by Steve Murray made a 50 ks observation of Cas A using the HRC-S
in the ``imaging" (aka ``timing") mode. An additional 50 ks observation
was made with the ACIS in order to obtain a spectrum of the source.
Fig. 5 shows an image of the central source.
The team, after extensive analysis of the data, found several plausible,
though not highly statistically significant, periodic signals. Using
spectral and luminosity
data and the age of the remnant to select the ``most probable of
the plausible", the team suggests a period of 12.155526 ms but emphasizes
that evidence for the existence of the pulsar is statistically weak. The
team has recently made a follow-up observation to try to determine the
true period, if one exists. Stay tuned. An ApJ article describing these
results will be published in February 2002 (preprint: astro-ph/0106516,
S. Murray, S. Ransom, M. Juda, U. Huang, and S. Holt)

Figure 6 on the left shows a 30 ks HRC-I observation
of M15 by Phil Charles and his team. M15 is perhaps the densest of all
globular star clusters in our Milky Way galaxy and has undergone a
process known as core collapse. The previously known source AC211
(4U2127), a low-mass X-ray binary (LXMB), is on the left, the ``other"
source is labeled XRB2. The width of the arrow is 2 arcsec. The image
on the right is a simulated image that was part of their original proposal.
The intensity ratio chosen for the simulation was 1:2, whereas the actual
measured ratio is about 1:3. The team concludes that this resolves the
``problem" of the X-ray bursts from M15, observed by the Japanese satellite
Ginga, which are almost certainly from XRB2. These two sources have also
been observed by Nicholas White and Lorella Angelini using ACIS/HETGS.

Figure 6:
The image on the left is an HRC-I image of M15 by
Phil Charles and his team. On the right is a simulation made
prior to the observation. See text for discussion.